"When Barack Obama was elected, they had enough fissile material, nuclear material, to make one bomb. Now they have enough to make five," Ryan said. "They're racing toward a nuclear weapon. They're four years closer toward a nuclear weapons capability."

The Wisconsin congressman said Iran's progress has sped along "because this administration has no credibility on this issue."

Biden hit back by criticizing what he called "bluster" and "loose talk" about the issue, saying international sanctions are crippling the Iranian economy and that U.S. and Israeli officials believe Iran is "a good way away" from getting the bomb.

The Vice Presidential Debate – An audience member holds a debate program.

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Photos:Photos: The Vice Presidential Debate

The Vice Presidential Debate – Debate moderator Martha Raddatz speaks to the crowd prior to the start of the vice presidential Debate.

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Iran has greatly expanded its ability to produce nuclear fuel in the past four years, revealing a second uranium enrichment plant in 2009 and continuing to defy U.N. demands that it halt work until questions about its intentions are resolved.

A bit of tech talk here: About three-quarters of 1% of naturally occurring uranium is uranium-235, the radioactive isotope used to produce a nuclear reaction. To produce fuel for nuclear power plants, that concentration has to be increased to 3 to 5%, while research reactors use fuel with a U-235 concentration of about 20%. To make a nuclear weapon, that concentration has to be increased to more than 90%.

As of May, Iran had produced about 6,200 kilograms (13,640 pounds) of power plant-grade uranium and 146 kilograms (320 pounds) of fuel at 20% concentration, according to the International Atomic Energy Agency, the U.N. nuclear watchdog.

Based on those figures, the Washington-based Institute for Science and International Security, estimated that Iran could produce enough weapons-grade uranium for five nuclear bombs. The organization, led by former U.N. weapons inspector David Albright, says Iran could build a crude bomb with 20-25 kg (44-55 pounds) of highly enriched uranium.

However, it would take Iran more than two months to produce that amount if it started with 20%-grade uranium, and "several months" to make enough for a bomb using low-enriched uranium. That would give the world community enough time to detect the operation and organize a response, ISIS noted in June.

And there's more to building a bomb than just having the uranium, as Biden noted. ISIS estimated last week, that Iran would need "many additional months" to develop a device that could be tested "and even longer to make a reliable warhead for a ballistic missile."

Iran has denied conducting any nuclear weapons research, insisting its nuclear program is for civilian purposes. The IAEA has verified that Iran has not diverted any of its declared uranium stock, but cautions that Iran "is not providing the necessary cooperation" to verify that all nuclear work is peaceful.

Israel has accused Iran of working toward a nuclear weapon, raising concerns that it could launch a preventive attack on Iranian nuclear facilities. Israel is believed to have its own nuclear arsenal, but has never disclosed whether it has the bomb -- and Iran has accused Israel and the United States of trying to sabotage its nuclear facilities.

As for the United States, American intelligence agencies believe Iran "is keeping open the option to develop nuclear weapons" but has not made the decision to build a bomb, Director of National Intelligence James Clapper told a Senate committee in January.

Conclusion: According to at least one widely circulated estimate, Iran could produce enough uranium for five bombs, as Ryan says. But the same estimate notes that it would draw attention if it did so, and that's only part of the work necessary to make a weapon.